Feat/task scheduling

[abelfx]

This pull request adds a global write lock to the backend to ensure only one write operation (such as CLEAR, UPLOAD, COMPOSITION, etc.) is processed at a time, and updates the frontend to display a "Queued..." status when a command is waiting for the lock. It also improves event messages for more accurate UI updates and user feedback.

Backend: Write Lock Implementation and Integration

• Introduced a global write lock using tokio::sync::Mutex in the backend, ensuring only one write operation can be processed at a time. The lock is shared via the SseState struct and acquired in all relevant routes before starting a write job (api/src/routes/sse.rs, api/src/lib.rs, api/src/routes/spaces.rs, api/src/scheduler.rs). - When the lock is busy, a log message ("Write queue busy, waiting for turn...") is broadcast to the frontend so the UI can indicate the queued state (api/src/scheduler.rs).

Backend: Event and Broadcast Improvements

• Enhanced SSE event messages to include the command name for both success and error events, allowing the frontend to accurately track which command completed (api/src/sse_utils.rs, api/src/routes/spaces.rs).

Frontend: Queued State Display

• Added logic to detect when a command is queued (waiting for the write lock) and display "Queued..." on action buttons in the UI for Clear and Composition pages (frontend/src/lib/sse.ts, frontend/src/pages/clear/Clear.tsx, frontend/src/pages/clear/lib.ts, frontend/src/pages/composition/Composition.tsx, frontend/src/pages/composition/lib.ts).
• Updated frontend event handling to parse the new event message format and only clear the active command when the correct command completes or fails (frontend/src/lib/sse.ts).

These changes improve consistency, prevent concurrent write issues, and provide clearer feedback to users about the status of their requests.